The utilization of ring laser gyro wherein two beams of coherent light are propagated in opposite direction within a laser cavity is well known in the prior art. See, U.S. Pat. No. 3,382,758, issued May 14, 1968, entitled "Ring Laser Having Frequency Offsetting Means Inside Optical Path," by Chas C. Wang. The laser cavity is generally formed in a polygonal configuration, such as a triangle or quadrilateral, about a rotation sensitive axis perpendicular to the plane formed by the polygon.
A suitable laser medium produces two beams of coherent light which travel in opposite path directions, as stated above. Each beam of light returns to the starting point so that rotation of the ring laser about its axis will cause the beam travelling in the same direction as that rotational motion to traverse a longer path, while the beam travelling in the opposite direction traverses a shorter path. The difference in path lengths traversed by the two beams creates a different frequency between the two beams in proportion to the rotational rate of the ring laser cavity. The counter-rotating beams are then permitted to exit the ring laser cavity where they are heterodyned to extract a beat note. The rotational rate of the ring laser gyro can thus be determined by the frequency of the beat note which is directly proportional to the aforesaid rotation rate.
If the ring laser gyro is to be used within an aircraft, for example, it will be subject to variations of the earth's magnetic field as that aircraft flies from one point to another. These changes in the magnetic field cause a nonreciprocal phase shift between the counter-rotating beams which appears as a frequency change in the beat note.
The prior art recoqnized the susceptibility of a ring laser gyro to the earth's magnetic field. One prior art patent taught that it was possible to eliminate the sensitivity of a ring laser gyro to magnetic field by exposing the gyro to a biasing magnetic field. See, U.S. Pat. No. 4,213,705, issued July 22, 1980, entitled "Four Mode Zeeman Laser Gyroscope With Minimum Hole Burning Competition," by Virgil E. Sanders, at lines 30-39. Another invention by Virgil E. Sanders, Ser. No. 443,057, filed Nov. 19, 1982 as a continuation of an earlier filed application filed on June 2, 1980, entitled "Zeeman Multioscillator Ring Laser Gyro Insensitive To Magnetic Fields And Deturning Frequencies," discloses the utilization of a particular combination of laser medium isotopes to create a ring laser gyro that is insensitive to magnetic fields. Each of the approaches taught by the two patents referred to hereinabove has merit. However, there are other solutions to this problem.